Chapter 7
Photosynthesis: Using Light to
Make Food
PowerPoint Lectures for
Biology: Concepts & Connections, Sixth Edition
Campbell, Reece, Taylor, Simon, and Dickey
Lecture by Richard L. Myers
Copyright © 2009 Pearson Education, Inc.
Light
energy
6 CO2 + 6
H2O
Carbon dioxide Water
C6H12O6
Photosynthesis
+ 6
O2
Glucose Oxygen gas
AN OVERVIEW
OF PHOTOSYNTHESIS
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7.1 Autotrophs are the producers of the biosphere
 Autotrophs - able to make their own food
– plants, algae and other protists, and
some prokaryotes
– Some use light, some use chemicals
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7.1 Autotrophs are the producers of the biosphere
– Chloroplasts - organelles consisting of
photosynthetic pigments, enzymes, and
molecules grouped together in
membranes – site of photosynthesis
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7.2 Photosynthesis occurs in chloroplasts in plant
cells
– Chlorophyll
 light absorbing pigment
 responsible for the green color of
plants
 converts solar energy to chemical
energy
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7.2 Photosynthesis occurs in chloroplasts in plant
cells
 Found in the mesophyll, the green tissue
in the interior of the leaf
 Stomata -tiny pores in the leaf that allow
carbon dioxide to enter and oxygen to exit
 Veins in the leaf deliver water absorbed by
roots
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Leaf Cross Section
Leaf
Mesophyll
Vein
CO2 O2 Stoma
Mesophyll Cell
Chloroplast
Experiment 1
6 CO2 + 12 H2O
Experiment 2
6 CO2 + 12 H2O
C6H12O6 + 6 H2O + 6 O2
Not
labeled
C6H12O6 + 6 H2O + 6 O2
Labeled
Reduction
6 CO2 + 6 H2O
C6H12O6 + 6 O2
Oxidation
Oxidation
C6H12O6 + 6 O2
6 CO2 + 6 H2O
Reduction
THE LIGHT REACTIONS:
CONVERTING SOLAR ENERGY
TO CHEMICAL ENERGY
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Increasing energy
10–5 nm 10–3 nm
Gamma
rays
X-rays
103 nm
1 nm
UV
106 nm
Infrared
103 m
1m
Microwaves
Radio
waves
Visible light
380 400
600
500
Wavelength (nm)
700
650
nm
750
Light and Pigments
 Chlorophyll does not absorb light well in the green region of
the spectrum. Green light is reflected by leaves, which is why
plants look green.
Estimated Absorption (%)
100
80
60
40
Chlorophyll
b
Chlorophyll
a
20
400 450 500 550 600 650 700 750
0
Wavelength (nm)
Light
Reflected
light
Chloroplast
Absorbed
light
Thylakoid
Transmitted
light
7.7 Photosystems capture solar power
 Pigments in chloroplasts are
responsible for absorbing
photons (capturing solar
power), causing release of
electrons
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e–
Excited state
Heat
Photon
Photon
(fluorescence)
Ground state
Chlorophyll
molecule
7.7 Photosystems capture solar power
 The energy released is conserved as it is
passed from one molecule to another
molecule- called photosystems.
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7.7 Photosystems capture solar power
 Two types of photosystems have
been identified and are called
photosystem I and photosystem II
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7.8 Two photosystems connected by an electron
transport chain generate ATP and NADPH
 During the light reactions, light energy is
transformed into the chemical energy of ATP and
NADPH
– electrons removed from water pass from photosystem
II to photosystem I and are accepted by NADP+
– The bridge between photosystems II and I is an
electron transport chain that provides energy for the
synthesis of ATP
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7.8 Two photosystems connected by an electron
transport chain generate ATP and NADPH
 NADPH, ATP, and O2 are the
products of the light reactions –
called the light dependent
reactions
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7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 Photosynthesis creates
concentration gradient of H+ ions
which powers the production of
ATP
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7.9 Chemiosmosis powers ATP synthesis in the
light reactions
 ATP synthase couples the flow of H+ to the
phosphorylation of ADP
– The chemiosmotic production of ATP in photosynthesis
is called photophosphorylation
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Chloroplast
Stroma (low H+
concentration)
Light
+
H+
Light
H
ADP + P
NADP+ + H+
H+
ATP
NADPH
H+
Thylakoid
membrane
H2O
1

2
O 2 + 2 H+
H+
Photosystem II Electron
transport
chain
Thylakoid space
+
(high H concentration)
H+
H+
H+
H+
H+
Photosystem I
H+
H+
H+
H+
ATP synthase
THE CALVIN CYCLE:
CONVERTING CO2 TO SUGARS
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7.10 ATP and NADPH power sugar synthesis in
the Calvin cycle
 The Calvin cycle makes sugar within a
chloroplast
– Need CO2
– ATP
– NADPH
– Called the light independent cycle
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Step 1 Carbon fixation
Input:
3
CO2
Rubisco
1
Step 2 Reduction
P
3 P
P
6
RuBP
3-PGA
6
3 ADP
ATP
6 ADP + P
Step 3 Release of one
molecule of G3P
3
ATP
CALVIN
4
2
CYCLE
6 NADPH
6 NADP+
Step 4 Regeneration of RuBP
P
5
P
6
G3P
G3P
3
Output: 1
P
G3P
Glucose
and other
compounds
PHOTOSYNTHESIS REVIEWED
AND EXTENDED
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H2O
CO2
Chloroplast
Light
NADP+
ADP
+ P
Photosystem II
Thylakoid
membranes
RuBP
CALVIN
CYCLE 3-PGA
(in stroma)
Electron
transport
chains
Photosystem I
ATP
NADPH
Stroma
G3P
O2
Sugars
LIGHT REACTIONS
CALVIN CYCLE
Cellular
respiration
Cellulose
Starch
Other organic
compounds
7.12 EVOLUTION CONNECTION: Adaptations
that save water in hot, dry climates evolved in
C4 and CAM plants
 Some plants have evolved a means of carbon
fixation that saves water during photosynthesis
– C4 plants because they first fix carbon dioxide into a
four-carbon compound – (sugar cane, corn, some
grasses)
– CAM – open stomata at night (cactus and pineapple)crassulacean acid metabolism
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PHOTOSYNTHESIS,
SOLAR RADIATION,
AND EARTH’S ATMOSPHERE
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7.13 CONNECTION: Photosynthesis moderates
global warming
 The greenhouse effect results from solar energy
warming our planet
 Global warming – the steady rise in temperature
of the earth caused by increasing amount of
greenhouse gases
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Some heat
energy escapes
into space
Sunlight
Atmosphere
Radiant heat
trapped by CO2
and other gases
7.14 TALKING ABOUT SCIENCE: Mario
Molina talks about Earth’s protective ozone
layer
– Ozone provides a protective layer
(the ozone layer) in our atmosphere
to filter out powerful ultraviolet
radiation
– industrial chemicals called
chlorofluorocarbons, or CFCs, deplete
the ozone layer
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Southern tip of
South America
Antarctica